Contact system for a high-voltage circuit breaker

ABSTRACT

A contact system for a high-voltage circuit breaker of the compressed-gas type or the like defines a gap across which an arc is drawn when the breaker is switched to the open position. The contact system includes at least one hollow, nozzle-shaped contact piece which defines one end of the gap when the breaker is switched to the open position. The contact piece includes an electrically conductive tubular member made of cold work-hardened copper. A nozzle-shaped body made of arc-resistant graphite is cold press-fitted into the tubular member at an end portion thereof so as to be held in surrounding engagement thereby in a friction-tight manner.

United States Patent [191 Marin CONTACT SYSTEM FOR A HIGH-VOLTAGECIRCUIT BREAKER [75] Inventor: Heiner Marin, Berlin, Germany [73]Assignee: Siemens Aktiengesellschaft,

Munich, Germany [22] Filed: June 4, 1973 [21] Appl. N0.: 366,541

[30] Foreign Application Priority Data June 20, 1972 Germany 2230927[56] References Cited UNITED STATES PATENTS l/l952 Harman 200/166 B10/1965 Gonek et al. 200/146 R FOREIGN PATENTS OR APPLICATIONS France200/148 G [111 3,835,274 Sept. 10, 1974 5/1968 Germany 200/148 R 8/1940Great Britain 200/148 G [5 7] ABSTRACT A contact system for ahigh-voltage circuit breaker of the compressed-gas type or the likedefines a gap across which an arc is drawn when the breaker is switchedto the open position. The contact system includes at least one hollow,nozzle-shaped contact piece which defines one end of the gap when thebreaker is switched to the open position. The contact piece includes anelectrically conductive tubular member made of cold work-hardenedcopper. A nozzleshaped body'made of arc-resistant graphite is coldpress-fitted into the tubular member at an end portion thereof so as tobe held in surrounding engagement thereby in a friction-tight manner.

4 Claims, 1 Drawing Figure PATENTEB SEPT 01974 3,835.2T&

CONTACT SYSTEM FOR A HIGH-VOLTAGE CIRCUIT BREAKER BACKGROUND OF THEINVENTION Deutsche Auslegungsschrift No. 1,154,548 discloses an electriccircuit breaker wherein an electro-negative, fluorine containing gassuch as sulphur hexafloride or the like is utilized as the quenching andinsulating medium. The circuit breaker includes two contact pieces whichare movable relative to each other and across which an arc is drawn whenthe breaker is opened. The contact pieces are made of carbon, preferablygraphite, in order to keep the destruction of the contact and theformation of decomposition products low during the time that the arc iseffective. The graphite contact pieces are solid and are inserted intocup-shaped electrodes. The walls of the cup-shaped electrodes engage atleast partially with undercuts on the periphery of the contact pieces toachieve a secure mechanical and electrical connection.

In high-voltage power circuit breakers, and in particular, incompressed-gas circuit breakers, it is advantageous to make the contactpieces hollow and suitable for exhausting the arc gases which areproduced during the opening operation of thebreaker. The hollow contactpieces have end faces whereat they carry the arcing electrodes. Asdisclosed in United States Patent application, Ser. No. 348,288, filedApr. 5, 1973, these arcing electrodes can have an annular configurationand can be provided with a U-shaped profile. A supporting body isarranged between the legs of the U- shapcd profile. In this embodiment,the graphite electrode is held in a friction-tight manner so as to besecured against breakage and to be mechanically stable.

United States Patent application, Ser. No. 344,045 filed Mar. 22, 1973,discloses how an annular graphite electrode is held by an annularelectrically conductive member in a friction-tight manner. The annularmember is in turn joined tightly to the corresponding contact piece andthe graphite member is configured to have a nozzle-like shape forconducting away the switching gases. The intermediate member which holdsthe electrode in place in this arrangement can be chosen with respect tothe currents and the material composition in such a manner thatfavorable coefficients of expansion relative to the graphite electroderesult. The graphite electrode must withstand high thermal stressbecause of the arc.

The arrangements disclosed in the two abovementioned Patent Applicationsrequire press fits in which the graphite nozzle as the inserted part iscooled I below the surrounding metal annular member while the annularmember is brought to a high temperature.

It is desirable to make the contact piece supporting the graphiteelectrode of a high electrically conductive material, particularlycopper, so that high rated currents can be transmitted withoutappreciable temperature rise. The use of copper tubing as the holder forthe graphite electrodes, however, causes problems because, with theinterference required for the friction fit, it is possible that thestrength of the copper can be exceeded. A shrink joint can thereforeproduce a tear.

SUMMARY OF THE INVENTION It is an object of the invention to join thegraphite electrode to the copper member in a contact system forhigh-voltage power circuit breakers. Subsidiary to this object, it is anobject of the invention to provide a friction-tight joint between thegraphite electrode and copper member which overcomes the disadvantagesdelineated above.

A contact system according to the invention is utilized in ahigh-voltage circuit breaker of the compressed-gas type or the like. Thecontact system defines a gap which is blasted with gas to quench an arcdrawn thereacross when the breaker is switched to the open position. Thecontact system includes at least one hollow, nozzle-shaped contact piecedefining one end of the gap when the breaker is switched to the openposition. According to a feature of the invention, the contact pieceincludes an electrically conductive tubular member made of coldwork-hardened copper and a nozzle-shaped body made of arc-resistantgraphite cold press-fitted into the work-hardened tubular member at theend portion thereof so as to be held in surrounding engagement therebyin a friction-tight manner.

The nozzle-shaped body consisting of graphite is thus pressed into thetube in cold condition and is embraced by the latter so as to form afriction fit therewith. The cold work-hardening of the copper tubecauses the strength properties thereof to be substantially increased;these properties are maintained below the recrystallization temperatureof the copper material used in each case, for instance, +l50C. Thecopper tube can consequently withstand the forces safely, which arerequired for a friction fit. With the invention, a joint is achievedwhich securely joins the copper and graphite members with each other.

In one preferred embodiment the peripheral surface intended for thefriction fit with the tube is formed by a cylindrical recess of thenozzle-shaped body which engages with a matching internal cylindricalrecess of the tube. The radial dimension of the internal recess of thetube can be about one-half of the wall thickness of the tube.

An amount of interference of the outer surface of the nozzle-shaped bodyrelative to the tube recess is preferably provided such that in atemperature range between 40 and +C, the pressure stresses of thematerial of the nozzle-shaped body and of the tube material do notexceed the permissible value at any point of the engagement area. Inorder to prevent chipping 0r cracking of the nozzle-shaped body, it isadvantageously stressed only in compression in the engagement area.

Although the invention is illustrated and described herein as a contactsystem for a high-voltage circuit breaker, it is nevertheless notintended to be limited to the details shown, since various modificationsmay be made therein within the scope and the range of the claims. Theinvention, however, together with additional objects and advantages willbe best understood from the following description and in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates the contactsystem according to the invention. The contact system is shown disposedin the switching chamber of an electric highvoltage power circuitbreaker of the compressed-gas type. The top half of the drawing abovethe center line shows the movable bridging member in a positioncorresponding to the closed position of the breaker, and in the lowerDESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The circuitbreaker has a switching chamber 2 consisting of porcelain, for example.The interior 1 of the chamber 2 is filled with sulfur hexafluoride asthe quenching and insulating medium. The switching chamber 2 is oftubular shape and includes two centrically disposed stationary contactpieces 3, 4; these contact pieces are mutually adjacent so as to definea gap therebetween and are of identical configuration. The contactpieces 3, 4 each include an electrically conducting tube 5, 6. Each ofthe tubes, 5, 6 has an end portion whereat a nozzle-shaped body 7, 8made of graphite is carried, the graphite being an arc-resistantmaterial. The two tubes 5, 6 are made of copper and are bridged in anelectrically conducting manner by a bridging contact member 12 when thebreaker is in the closed position as shown above the longitudinal centerline.

Through a movement of the bridging contact member 12 in the direction ofthe arrow 13, the bridging contact member 12 arrives in the positionshown below the center line which corresponds to the open position ofthe breaker. When the member 12 moves to the open position, an arc isdrawn between the graphite nozzle-shaped bodies 7 and 8. A jet of gasoriginating in the interior 1 of the switching chamber 2 drives the areinto the interior of the hollow, nozzle-shaped contact pieces 3 and 4until it is extinguished at the zero crossing of the alternatingcurrent.

The tubes 5 and 6 of the contact pieces consist of cold work-hardenedcopper and have each, in the area of the end portion, a recess in theform of a cylindrical counterbore 9, the radial dimension of which isabout one-half of the wall thickness a of the tubes 3, 4. Thenozzle-shaped bodies 7, 8 each have a cylindrical open recess 11 attheir outer surface 10 which establishes a friction fit with thecorresponding tube 5 or 6. The nozzle-shaped bodies 7, 8 consisting ofgraphite are cold pressed into the counterbore 9 of the tubes 5 and 6 soas to leave part of the nozzle-shaped bodies 7, 8 protruding. During thepressing operation, grooves are produced in the graphite body at thecylindrical surface 11 through scoring action. The material from thesegrooves is present, for instance, in the form of powder and can serve asa lubricant during the pressing operation.

The configuration of the tubes 5, 6 with a counterbore 9 relieves thenozzle-shaped bodies of load in the region of the unengaged end of thebody where a fracture could occur; this configuration also ensures agreater area over which pressure is applied at the engaged end of thenozzle-shaped body because there, the recessed region of the tube isaided by the adjacent bounding portion of the tube.

For the surface portion of the nozzle-shaped body 10, 11 such an amountof interference relative to the tube counterbore 9 is preferably chosenso that in a temperature range between 40 and-+1 C, the permissibletensile stresses for the tube material (copper) and the permissiblecompression stresses for the material of the nozzle-shaped body(graphite) are not exceeded at any point of the engagement areawherewith the area pressure in the engagement area does not fall belowthe required value.

In a preferred embodiment, the following data are obtained:

Tube, tensile stress 25 kp/mm Nozzle-shaped body, compression stress 4kp/mm Area pressure 0.4l.2 kp/mm Interference approx. 2.5%

With the invention, it is possible to configure the part of the contactpiece holding the nozzle-shaped graphite body of a material favorablefor carrying rated current of the power circuit breaker; such a materialis copper and the required strength properties are achieved by coldwork-hardening. This ensures that the nozzleshaped body is subjected inthe engagement area to a pressure that assures a friction fit withoutthe copper flowing in the engagement area. Making the joint requires noparticular heat treatment and no subsequent finishing of the joinedparts is needed.

Although the contact system is described in the context of high-voltagepower circuit breakers and particularly compressed-gas circuit breakers,it can be applied to other high-voltage equipment provided with agaseous insulating medium. Thus, it is, for example, possible to equipload disconnect switches with the contact system according to theinvention. It is also possible to equip overvoltage arresters with thecontact system of the invention at the gas-filled spark gaps thereof.

What is claimed is:

l. A contact system for a high-voltage circuit breaker of thecompressed-gas type or the like, the contact system defining a gap whichis blasted with the gas to quench an arc drawn thereacross when thebreaker is switched to the open position, the contact system comprisingat least one holow, nozzle-shaped contact piece defining one end of thegap when the breaker is switched to the open position, said contactpiece including an electrically conductive tubular member made of coldwork-hardened copper, said tubular member having end portion, anozzle-shaped body made of arc-resistant graphite, said nozzle-shapedbody being cold press-fitted into said tubular member at said endportion thereof so as to be held in surrounding engagement thereby in afriction-tight manner.

2. The contact system of claim 1, said tubular member having acounterbore formed in the inner surface thereof at said end-portion, andsaid nozzle-shaped body having an open cylindrical recess adapted tosaid counterbore and formed in its outer surface for engaging saidtubular member in a friction-tight manner at said counterbore.

3. The contact system of claim 2, said counterbore being a recess in theinner surface of said end-portion, said recess having a dimensionmeasured in radial direction equal to approximately one-half thethickness of said tubular member beyond said end portion.

4. The contact system of claim 2, said nozzle-shaped body beingdimensioned to have an amount of oversize of said cylindrical surfacerelative to said counterbore so as to cause the compressive stressdeveloped in the material of said nozzle-shaped body and the tensilestress developed in said tubular member to remain within the respectiveallowable values therefor at all points of the engagement area of saidnozzle-shaped body and said tubular member in the temperature range of40C to +C.

1. A contact system for a high-voltage circuit breaker of thecompressed-gas type or the like, the contact system defining a gap whichis blasted with the gas to quench an arc drawn thereacross when thebreaker is switched to the open position, the contact system comprisingat least one holow, nozzle-shaped contact piece defining one end of thegap when the breaker is switched to the open position, said contactpiece including an electrically conductive tubular member made of coldwork-hardened copper, said tubular member having end portion, anozzle-shaped body made of arc-resistant graphite, said nozzle-shapedbody being cold press-fitted into said tubular member at said endportion thereof so as to be held in surrounding engagement thereby in afriction-tight manner.
 2. The contact system of claim 1, said tubularmember having a counterbore formed in the inner surface thereof at saidend-portion, and said nozzle-shaped body having an open cylindricalrecess adapted to said counterbore and formed in its outer surface forengaging said tubular member in a friction-tight manner at saidcounterbore.
 3. The contact system of claim 2, said counterbore being arecess in the inner surface of said end-portion, said recess having adimension measured in radial direction equal to approximately one-halfthe thickness of said tubular member beyond said end portion.
 4. Thecontact system of claim 2, said nozzle-shaped body being dimensioned tohave an amount of oversize of said cylindrical surface relative to saidcounterbore so as to cause the compressive stress developed in thematerial of said nozzle-shaped body and the tensile stress developed insaid tubular member to remain within the respective allowable valuestherefor at all points of the engagement area of said nozzle-shaped bodyand said tubular member in the temperature range of -40*C to +120*C.